Modern technology has produced a wide variety of electronic devices adapted to indicate the bearing and distance of an object from a vehicle. Examples of relatively broad categories of such devices are radio direction finders, target tracking radars, storm cloud tracking radars and atmospheric disturbance detectors.
Radio direction finders generally incorporate a simple means to indicate bearing only except in the case of target identification transpondors. These latter systems function in cooperation with a rotatable antenna synchronized to a plan position indicator or PPI oscilloscope adapted to function similar to a tracking radar system except a transpondor is located in the target and adapted to provide a reply which is utilized by the system instead of a radar echo.
Radar systems using PPI displays have been used in vehicles such as aircraft for a significant number of years to indicate a large variety of items such as land masses, cities, other aircraft and areas of heavy precipitation. This latter application of radar systems commonly known as weather radar has found wide spread use in aircraft due to its ability to aid a flight crew in avoiding severe turbulence associated with thunderstorms.
A relatively newer approach to detecting severe turbulence associated with thunderstorms has recently been provided by systems combining radio receivers and plan position indicators wherein the radio receivers are responsive to the electromagnetic energy generated by lightning.
All of the foregoing systems utilize plan position indicators based upon cathode ray tubes which incorporate an electronically produced radial deflection in combination with an electromechanically produced rotational deflection of an electron beam. The rotational deflection or sweep of the beam is generally produced by rotating a deflection coil about the neck of a cathode ray tube in synchronization with a rotating or oscillating antenna. More recent technology has produced systems in which the target data is stored in computer means and displayed on the face of the cathode ray tube as a result of stationary electronic deflection means responsive to bearing related address data for the target data in storage.
All of the above plan position indicating devices have a serious drawback when used in a vehicle because the target data is the result of the instantaneous relative bearing at the time the signal is received. This vehicle heading oriented display remains fixed so that when the vehicle turns, the target image which has been retained fails to reflect the change in bearing from the vehicle. This results in enlarged and distorted targets and erroneous targets when high rates of turn are encountered.
In the atmospheric disturbance detection devices utilized to detect the presence of thunderstorms, the display means is usually a computer processed relative bearing indication on a cathode ray tube utilizing electronic deflection only. In these systems the display is a function of an automatic direction finding signal processed by a phase responsive antenna system. The received signals are retained in a storage means for a relatively long period of time when contrasted to normal radar return echos and used to generate a picture of severe weather cells. If the vehicle turns, the bearing data presented will be in error as a function of the amount of heading change of the vehicles since the last display update. This could be as much as 180 degrees in some instances and render the display completely useless.